The invention relates to feedback loop control systems. More particularly, it relates to feedback loop control systems for machine tool slide drivers employing a position feedback control loop in combination with an optional tachometer feedback control loop.
In general, a servomechanism positioning system utilizes a closed-loop, automatic feedback control system in which the position of the load or the state of the controlled quantity is sensed or measured and then fed back for comparison between the actual position (or the actual state) and the desired position. This difference is used to regulate the output of the servomechanism. If the feedback signal and the reference input signal are not equal, a deviation or actuation signal is produced which, after suitable amplification, is impressed on the servo actuator, usually a servomotor. The servomotor, in turn, corrects the output variable so as to bring the feedback signal into coincidence with the reference input signal. System response, or the actual real-time position of a servoed device, is always different from command position in a highly variable manner. This is unavoidable since response is a complex function of the following system parameters: (1) command speed or position, per se, (2) the magnitude of incremental changes in command speed or position, (3) elapsed time from latest changes in command speed or position, (4) instantaneous bandwidth of the servoed device's governing control loop or correction constant, (5) instantaneous magnitude of time-dependent forward gain from the command input to the servo actuator, (6) command input update and hold times, (7) parameter tolerance errors, and (8) degradation of response agility from lost bandwidth as the command input approaches zero. The command-to-response error is greatly magnified when two or more servos operate together in real time, particularly when widely different command inputs are applied to the individual servo systems. Such a servo combination, nevertheless, is typically incorporated in machine tool slide drives employed in contour cutting operations. Thus, if servo-caused cutting errors are to be limited to magnitudes measured in microinches, or even sub-microinches, conventional closed loop servo correction and control design methods and hardware for minimizing command-to-response error, which is a function of the system parameters noted above, become increasingly inadequate.
Various approaches have been undertaken in attempting to reduce or stabilize this command-to-response error, or servo following error, while avoiding either reducing servo positioning speed or imposing prohibitive bandwidth operating limitations as discussed above. One approach is the subject of U.S. Pat. No. 3,798,430. The invention described therein involves controlling the velocity of a movable member along a given axis X by updating position commands by increments of .DELTA.X during successive equal time periods .DELTA.T so as to move the controlled member along the X-axis at a velocity V.sub.x =.DELTA.X/.DELTA.T. The changing position command signal is periodically incremented by variable amounts during each of successive, equal time periods, .DELTA.T, in order to produce feed forward signals which are proportional to the individual axis velocities at which the controlled member is to be moved, so as to make its resultant velocity equal to that designated by numerical program information. This feed forward approach suffers from inherent performance limitations in that position corrections are determined with respect to the commanded position rather than the response position and/or velocity of the controlled member. Another approach is described by R. Palmer, Control Engineering, page 53, March 1978, which involves feeding the position error around the lead-lag compensation networks of the servo control loop and through a non-linear gain circuit. Although reductions in servo following error are realized by this technique, it fails to adequately address and compensate for the many factors which give rise to command-to-response errors. In addition, for large position errors, correction signals are limited in magnitude to a nearly constant voltage which was only 10-25% of full scale output. The present invention, however, overcomes all of the foregoing limitations by basing position correction errors on the response position of the controlled element, and by generating a greatly amplified position correction signal (to the full slew mode of operation) for even the smallest command-to-response position errors while immediately reverting to conventional control when the error signal is driven to zero.
It is an object of the present invention to reduce the response time of a servo positioning system by 2-3 orders of magnitude over that of a conventional servo system when input command signal levels, or changes therein, are very small.
Another object of the invention is to increase servo system instant positioning accuracy by operating the position feedback loop at a higher clocking frequency than the command update rate thus making accuracy independent of data sampling rates and associated system clocking frequencies.
Still another object of the invention is to reduce following error in a servo positioning system without increasing operating bandwidth by providing greatly amplified position correction signals for large position corrections while small input corrections are provided by more conventional feedback means.
Other objects and advantageous features of the invention will be apparent in a description of a specific embodiment thereof, given by way of example only, to enable one skilled in the art to readily practice the invention which is described hereinafter with reference to the accompanying drawing.